ProPresenter handles the majority of media files as Backgrounds or Foregrounds. Media that is added to a slide in the Editor is excluded from this explanation.
Every media cue is marked with an icon in the top left corner of the thumbnail. These icons match the ones shown next to the Background and Foregrounds bins in the Video/Image Bin. Any media that is added directly to the program in the bins or added directly to a presentation from the computer will end up as a Background or Foreground. If you drag a media file from Finder or Windows Explorer directly into a presentation and drop it onto an existing slide, it will be added as a Background. If you drop the same cue between slides or into a presentation by itself, it will be added as a Foreground.
ProPresenter is built on a system of layers. In simple terms, you have the slide color, the Background media, the slide content, and Foreground media.
The simplest slide you can build is a solid color background (black by default) and a single text box. This uses just two layers. You can then add a Background--video or still image--to the cue. The Background is placed between the slide color and text box. A background will continue to show as you change slides, so you only need to add a background one time to a presentation. Slides can contain a number of slide objects, including additional pictures and videos, that will all display above the Background media. By default, all Backgrounds are set to Stretch to Fill so that they fill up the output. You can read more about the scaling options here.
Foregrounds are a unique layer. This layer is intended and designed to be shown by itself. If you have a background and slide on screen, a Foreground will clear the other layers. Foregrounds are generally used for special-purpose videos. They are also set to Scale to Fit, which may result in seeing black bars on two sides of your file if it is not the same aspect ratio as your output.
You can switch any media cue between being a Background or Foreground in the Video/Image Bin by right-clicking and selecting Set to Background or Set to Foreground. Adding media directly to the Background or Foreground bins will also set those files accordingly.